Velocity modulation tube having floating resonator circuits and short drift spaces

ABSTRACT

In a velocity modulation tube comprising at least one floating prebuncher resonator and at least one final floating resonator, in which at least one of the floating prebuncher resonators has a fundamental mode of resonance at a frequency lower than the center frequency of the operating passband of frequencies of the tube, the normalized length of the drift space placed downstream of each low frequency floating prebuncher resonator is made longer than the normalized lengths of the drift spaces located downstream of the other resonators and not longer than 90* in terms of the reduced plasma angle.

United States Patent 1191 Kageyama 1 June 25, 1974 [54] VELOCITY MODULATION TUBE HAVING 3,447,018 5/1969 Schmidt 315/5.52 X FLOATING RESONATOR CIRCUITS AND 3,594,606 7/1971 Lien 315/5.51 3,622,834 11/1971 Lien 315/5151 SHORT DRIFT SPACES [75] Inventor: Takao Kageyama, y Japan Primary Examiner.lames W. Lawrence 73 Assignee; Nippon Electric Company, Limited, Assistant Exammer-S axfield- Chatrnon, Jr.

Tokyo, Japan Attorney, Agent, or F1rmOstro1enk, Faber, Gerb & 221 Filed: Mar. 26, 1973 sffen [21] Appl. No.: 344,647 [57] ABSTRACT In a velocity modulation tube comprising at least one [30] Foreign Application Priority Dat floating prebuncher resonator and at least one final Apr. 18 1972 Japan 47-38897 floating resonator in which at least one of the floating prebuncherresonators has a fundamental mode of res- 52 us. c1 315/5.43, 315/551 3 15/552 ettettee at a frequency lower the eetttet 511 1111. C1. l-l 0lj 25/10 quehey of the Operating Petsthand of frequencies of [58] Field of Search 315/543 5.51 5.52 the "the, the normalized length of the drift Space placed downstream of each low frequency floating 56] References Cited prebuncher resonator is made longer than the normalized lengths of the drift spaces located downstream of UNITED STATES PATENTS the other resonators and not longer than 90 in terms 2,494,721 1/1950 Robertson 315/5.43 of the reduced plasma ang]e 3,195,007 7/1965 Watson et a1... 315/5.43 3,289,033 11/1966 Saburi 3l5/5.43 X 6 Claims, 6 Drawing Figures 1 17 a 2g 22 23; 26 I 3 /3 6256710 VELOCITY MODULATION TUBE HAVING FLOATING RESONATOR CIRCUITS AND SHORT DRIFI SPACES BACKGROUND OF THE INVENTION This invention relates to a velocity modulation tube comprising a plurality of intermediate or floating resonators and short drift spaces. The floating resonator of a velocity modulation tube is defined as a resonator which is placed intermediate the input and the output resonators and which neither has a source of energy external to the tube nor a load utilizing the output power of the resonator. This, however, does not preclude a circuit coupled to the resonator solely for effecting certain characteristics of the resonator.

In connection with a velocity modulation tube it is reminded here that the interaction gap associated with a resonator contributes to the interaction between the electron beam produced in the tube and the electromagnetic field induced in the resonator and that the drift space interposed between two adjacent interaction gaps contributes to bunching of the velocity modulated electron beam. A velocity modulation tube having at least one floating prebuncher cavity resonator and at least one final floating cavity resonator is known, in which the normalized lengths of the drift spaces are about 90 or less in terms of the reduced plasma angle. The normalized length of a drift space is given in terms of the reduced plasma angle by fl l/u where 0,, represents the reduced plasma angular velocity, u represents the DC. beam speed, and 1 represents the physical distance between the middle points of the interaction gaps placed at both ends of the drift space. For brevity, the phrase in terms of the reduced plasma angle will be omitted in the following where possible. In addition, a drift space located immediately downstream of the interaction gap associated with a resonator will merely be referred to as a drift space located immediately downstream of a resonator where intelligible. The words upstream and downstream refer to the macroscopic flow of the electron beam. It is also known to select various frequenices for the fundamen tal modes of resonance of the resonators in order to improve the gain-to-frequency characteristics of the tube. Thus, the floating prebuncher resonator placed next downstream of the input resonator is often possessed of a fundamental mode of resonance at a frequency lower than the center frequency of the operating passband of frequencies of the tube.

In a conventional velocity modulation tube of the kind described, it is usual that the normalized length of the first drift space located immediately downstream of the input resonator is about 90 in view of the power gain of the tube and in consideration of the fact that the voltage level produced across the interaction gap of the input resonator for velocity modulating the electron beam is of the small-signal nature. Furthermore, the normalized lengths of the second and the third drift spaces placed immediately downstream of the floating prebuncher an the final floating resonatoes, respectively, are usually selected in a range between about 60 and depending on the voltage levels produced across the interaction gaps of these resonators because these voltage levels approach a large signal. As a result of the computer simulation of the large-signal operation of a velocity modulation tube using a disk model it has been found in connection with a floating prebuncher resonator that debunching occurs in the bunched electron beam because the voltage induced across its interaction gap in the vicinity of the center frequency is approximately in phase opposition to the voltage produced across the interaction gap of the input resonator. Once the debunching occurs, the normalized length of the order of of the second drift space located immediately downstream of the floating prebuncher resonator is too short to rebunch the electron beam. This does not provide the desired electron bunching at the interaction gap of the output resonator and adversely affects the conversion efficiency of the tube.

In order to enhance the conversion efficiency, a velocity modulation tube is revealed in the United States patent application Ser. No. 28,791 filed Apr. 15, 1970, by Erling L. Lien, now U.S. Pat. No. 3,594,606 issued July 20, 1971, wherein it is intended to utilize the second harmonic in effecting strong electron bunching action by locating upstream of the final floating resonator a combination of two second harmonic resonators and a fundamental mode resonator interposed therebetween. The proposed velocity modulation tube, however, requires two redundant floating resonators. This lengthens the tube by about in terms of the normalized drift length and adds to the tube weight. As a result, the proposed tube has certain demerits, such as difficulty in handling and the resulting bulkiness of the UHF television transmitter equipped with the tube in its power amplifier stage.

In order to attain the bunching effected by the second harmonic with fundamental mode resonators rather than with the use of the two second harmonic resonators, another velocity modulation tube is disclosed in the United States patent application Ser. No. 28,792 filed Apr. 15, 1970, by Erling L. Lien, now U.S. Pat. No. 3,622,834 issued Nov. 23, 1971, wherein use is made of a lengthy drift space of the normalized length between 90 and 150, preferably of located between a floating prebuncher and the final floating buncher placed next downstream of the floating prebuncher. With this arrangement, second harmonic space charge forces produced in the lengthy drift space tend to drift the electrons placed in the interbunch region towards the centers of the electron bunch to bring about the desired strong bunching effect and to raise the conversion efficiency. It is to be pointed out, however, in connection with utilization of the second harmonic space charge forces that an abnormally long drift space of the normalized length of, for example, 120 is necessary which renders the tube lengthy as compared with a similar tube comprising drift spaces whose normalized lengths are about 90 or less. Alike the tube in which use is made of a combination of a fundamental mode resonator and a pair of second harmonic resonators, the tube having a lengthy drift space results in similar troubles, such as the difficulty in handling and the increased bulk of the UHF television transmitter equipped with the tube in the power amplifier. v

In summary, the improvement effected on the conversion efficiency by the bunching effect caused by the second harmonic requires either additional second harmonic resonators or an abnormally long drift space of the order of 120 which to result in a lengthy velocity modulation tube.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a high efiiciency velocity modulation tube of relatively short tube length.

It is another object of this invention to provide a high efficiency velocity modulation tube without resorting to the bunching action of the second harmonic.

It is still another object of this invention to provide a high efficiency velocity modulation tube in which the normalized drift lengths are not longer than 90 in terms of the reduced plasma angle.

In the manner known in the art, a velocity modulation tube according to this invention also comprises a plurality of floating resonator circuits placed intermediate an input and an output resonator circuit and a plurality of drift spaces for an electron beam produced in the velocity modulation tube, each resonator circuit comprising interaction gap means operatively associated therewith and contributing to interaction between the electron beam and electromagnetic field induced in the associated resonator circuit, each of the drift spaces extending between the interaction gap means associated withadjacent two of the resonator circuits. The floating resonator circuits comprises at least one first resonator circuit and at least one second resonator circuit placed downstream of the first resonator circuits in respect of the electron beam, at least one of the first resonator circuits being a low frequency resonator circuit tuned for a fundamental mode of resonance at a frequency lower than the substantial center frequency of the operating passband of frequencies of the velocity modulation tube. In accordance with this invention, the normalized length of drift space located immediately downstream of the interaction gap means associated with each of the low frequency resonator circuits is longer than the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than 90 in terms of the reduced plasma angle.

The resonance circuit and the associated interaction gap means may be typically a reentrant circular cylindrical cavity or, as the case may be, a combination of a plurality of radially symmetric cavity resonators aligned along a common axis, each having an interaction gap substantially along the axis thereof.

The short normalized drift lengths render the velocity modulation tube relatively short. The low frequency resonator circuits improve the gain-to-frequency characteristics of the tube. The longer normalized drift length immediately following each of the low frequency resonator circuits serves to re-bunch the debunched electrons thereby enhancing the conversation efficiency of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a schematic side view of a first embodiment of the instant invention;

FIG. 2 schematically shows bunching, debunching, and rebunching of the electrons carried out along the tube axis of a velocity modulation tube according to this invention;

FIG. 3 is a graph showing the impedance phase of a floating prebuncher used in a velocity modulation tube versus the operating passband of frequencies of the tube;

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Referring to FIG. 1, a first embodiment of the present invention comprises an electron gun assembly 11 for producing an electron beam 12, a collector electrode 13 disposed at the remote end, an input resonator circuit 14 placed at the upstream end of the beam 12 and having an input coupler 15 connected to an input energy source 16 and an interaction gap 17 for velocity modulating the beam 12 with the high frequency voltage applied across the gap 17 by the electromagnetic field induced in the input resonator circuit 14 by the energy supplied from the energy source 16, a first drift tube 18 placed immediately downstream of the input resonator circuit 14 and surrounding the beam 12 to provide a high frequency field free space in" which the electrons may drift with the velocities determined by the velocity modulation effected thereon by the input resonator circuit 14, a floating prebuncher 19 having a fundamental mode of resonance at a frequency lower than the center frequency of the operating passband of frequencies of the velocity modulation tube and having an interaction gap 20 at the downstream end of the first drift tube 18 and means, such as a coupler 21, for connection to a circuit element (not shown) for adjusting electric characteristics of the prebuncher 19, such as its Q or frequency, and a second drift tube 22 surrounding the beam 12in the region downstream of the gap 20 of the prebuncher 19 to provide another high frequency space of the normalized length longer than the first drift space and other drift spaces described later and which not longer than. While drifting through the first drift tube 18, the electrons are bunched to provide a density modulated component in the beam current at the gap 20 of the prebuncher 19 which has a phase lag of about 90 with respect to the voltage applied across the gap 17 of the input resonator circuit 14 and which energizes'the floating prebuncher 19 to induce along the resonator wall a current in phase with the density modulated current component. On the other hand, the impedance of the prebuncher l9 seen from its gap 20 in the vicinity of the center frequency is capacitive because the prebuncher 19 is a low frequency resonator circuit. The current induced along the resonator wall therefore induces a voltage across the gap 20 of the prebuncher 19 which further lags behind the wall current by about 90 of phase and is approximately in phase opposition to the voltage applied across the gap 17 associated with the input resonator circuit 14. As a result, the electrons are subjected to velocity modulation at the gap 20 of the low frequency resonator circuit 19 towards debunching and are substantiallly entirely debunched immediately after entering the second drift tube 22. It has now been found possible to rebunch the once debunched electrons during their passage through the second drift tube 22 of the characteristics mentioned above.

Further referring to FIG. 1, the first embodiment comprises a first final floating buncher 23 tuned for a fundamental mode of resonance at a frequency higher than the upper edge of the passband and having an in teraction gap 24 at the downstream and of the second drift tube 22, a third drift tube 25 surrounding the beam 12 in the region downstream of the gap 24 to provide a high frequency field free drift space, a second final floating buncher 26 similar to the first final buncher 23 and provided with an interaction gap 27 at the downstream end of the third drift tube'25, a fourth drift tube 28 similarly surrounding the beam 12 in the downstream region of the gap 27, and an output resonator circuit 29 having an interaction gap 30 at the downstream end of the fourth drift tube 28 and the output coupler 31 connected to a utilization device 32, such as an antenna, to supply thereto the amplified energy developed in the output resonator circuit 29. The third and the fourth drift spaces have shorter normalized lengths than the second drift space as has already been described above. Due to the higher frequency nature of the final floating bunchers 23 and 26, the impedance thereof seen from their respective gaps 24 and 27 in the passband of frequencies are sufficiently inductive. The voltage induced across the gap 24 or 27 by the density modulated current component of the beam 12 is thus approximately in phase with the voltage induced across the gap 20 of the floating prebuncher 19 and velocity modulates theelectrons to enhance the electron bunching in the second'drift tube 22. The density modulated current component of the beam 12 wherein the electron bunching is thus enhanced energizes the output resonator circuit 29 to provide the output energy.

By way of example, the normalized lengths of the drift spaces provided by he first, the second, the third, and the fourth drift tubes 18, 22, 25, and 28 are 50, 80, 45, and 33, respectively.

Referring now to FIG. 2, description will be given more in detail of the debunching and the re-bunching to which the electrons are subjected at the interaction gap 20 of the low frequency resonator circuit 19 and within the second drift tube 22. In FIG. 2, the DC. motion of the electrons is eliminated. In other words, FIG. 2 shows the manner in which the electrons of the beam 12 are subjected to velocity modulation and bunching as observed by the observer moving along the beam 12 at the DC. beam speed to enable the beam 12 to be represented with the time fixed rather than dealt with as a continuous flow dependent on both distance and time. In addition, the behavior of the electrons is exemplified by eight disks A through H placed initially at the respective octopartitions of that length of the'beam 12 which corresponds to one cycle of the oscillation of the center frequency of the passband. FIG. 2 ,(a) shows the high frequency produced across the gap 17 of the input resonator circuit 14, which velocity modulates the disks in the senses illustrated with arrows. FIG. 2 (b) shows the disks immediately downstream of the gap 17, on which the electrons are not yet bunched but have their respective velocities depicted with arrows. FIG. 2 c) shows the disks immediately upstream of the gap 20 of the low frequency resonator circuit 19, which have centers of the electron bunch at the positions of the disk A. In FIG. 2 (d), a curve 36 depicted with a dashed line shows the density modulated component of the beam 12 at the gap 20, which induces the in-phase wall current. Another curve 37 depicted with a solid line illustrates the voltage induced across the gap 20 together with the velocity modulation effected on the disks by the induced voltage. The velocity modulation indicated with arrows tends to debunch the electron bunch at the positions of the disk A and re-bunches the electrons towards the positions of the disk E which is positioned in the interbunch regionsimmediately upstream of the gap 20. FIG. 2 (e) shows the electrons substantially completely debunched in the immediately downstream region of the gap 20. The electrons, however, are possessed of velocities modulated by the voltage induced across the gap 20 to tend towards the disk E as shown with arrows. FIG. 2 (f) shows the-electron re-bunching effected within the second drift tube 22. his thus possible according to this invention to re bunch the electrons to raise the conversion efficiency with the second drift tube 22 of the normalized length shorter than in contrast to the fact that a drift space of the normalized length of the order of is necessary when use is made of the second harmonic space charge forces to achieve enhancement of the electron bunching for raising the conversion efficiency.

Referring to FIG. 3, the curve depicted therein shows the phase of the impedance of the low frequency resonator circuit 19 seen from its gap 20 versus frequencies. In FIG. 3, f, represents the center frequency of the passband and 1" shows a frequency at which the low frequency resonator circuit 19 may be tuned for the fundamental mode of resonance to improve the passband characteristics of the velocity modulation tube. It is readily seen that the phase of the impedance lags about 90 in the vicinity of the center frequency 1",.

Referring to FIG. 4, the curve illustrated therein represents the gain of a velocity modulation tube according to the first embodiment versus the frequency scaled in MHz. The center frequency of the passband is 600 MHz. The frequencies at which the respective resonator circuits 14, 19, 23, 26, and 29 are turned for their fundamental modes of resonance are shown by the re spective reference numerals. These frequencies furnish the velocity modulation tube with an operating passband of frequencies about 7 MHz wide between the points having 1 dB less the maximum gain. The frequency for the low frequency resonator circuit 19 is a little lower than the lower edge of the passband. The frequencies for the final floating resonator circuits 23 and 26 are higher than the upper limit of he passband. The frequencies for the input and the output resonator circuits l4 and 29lie in the vicinity of the center'frequency.

Referring to FIG. 5, a curve 38 shows the amplitude of the fundamental component of the density modu lated current as normalized by the DC. beam current versus the distance measured in mm-from the most upstream interaction gap 17 along the path of the beam 12 for a velocity modulation tube according to the first embodiment. The positions of the gaps 17, 20, 24, 27, and. 30 are indicated by the respective reference numerals. Another curve 39 depicted generally below the curve 38 shows the normalized density modulated current versus the distance for a conventional velocity modulation tube having the same distance between the most upstream and downstream gaps and provided with 7 a long drift space immediately downstream of the input resonator circuit. The middle point of the gap located immediately downstream of the long drift space is represented with a letter X. The re-bunching achieved in the drift tube 22 according to the present invention is strong enough to raise the conversion efficiency about /o.

Referring to FIG. 6, a second embodiment of the invention is similar to the first embodiment except for substitution of only one final floating buncher for the two final floating bunchers. The second embodiment comprises parts which are merely indicated with like reference numerals without further description indicated with like reference numerals without further description herein. An example of the actual normalized lengths of the drift spaces provided by the drift tubes 18, 22 and 28 are 50, 80, and 35, respectively.

What is claimed is:

l. A velocity modulation tube operable in a predetermined operating passband of frequencies comprising at least one first resonator circuit and at least one second resonator circuit each located between an input and an output resonator circuit and a plurality of drift spaces for an electron beam passing through all of said resonator circuits and said drift spaces, said second resonator circuit being placed downstream of said first resonator circuit with respect to said electron beam, each of said resonator circuits comprising interaction gap means operatively associated therewith and contributing to interaction between said electron beam and the electromagnetic field induced in the associated resonator circuit, selected ones of said drift spaces extending between the interaction gap means associated with two adjacent ones of said resonator circuits, said first resonator circuit having a fundamental mode of resonance at a frequency lower than the substantial center frequency of said passband, wherein the improvement is such that the normalized length of the drift space placed immediately downstream of the interaction gap means associated with said first resonator circuit is longer than the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than 90 in terms of the reduced plasma angle.

2. A velocity modulation tube as claimed in claim 1, wherein'said second resonator circuit has a fundamental mode of resonance at a frequency higher than said center frequency.

3. A velocity modulation tube as claimed in claim 2, wherein said first resonator circuit has a fundamental mode of resonance at a frequency lower than the lower edge of said passband.

4. A velocity modulation tube as claimed in claim 2, wherein said second resonator circuit has a fundamental mode of resonance at a frequency higher than the upper edge of said passband.

5. A velocity modulation tube operable in a predetermined operating passband of frequencies comprising at least one first resonator circuit and at least one second resonator circuit each located between an input and an output resonator circuit and a plurality of drift spaces for an electron beam passing through all of said resonator circuits and said drift spaces, said second resonator circuit being placed downstream of said first resonator circuit with respect to said electron beam, each of said resonator circuits comprising interaction gap means operatively associated therewith and contributing to interaction between said electron beam and the electromagnetic field induced in the associated resonator circuit, selected ones of said drift spaces extending between the interaction gap means associated with two adjacent ones of said resonator circuits, said first reso nator circuit having a fundamental mode of resonance at a frequency lower than the substantial center frequency of said passband, wherein the improvement is such that the normalized length of the drift space placed immediately downstream of the interaction gap means associated with said first resonator circuit is longer than the normalized lengths of the drift spaced placed irrunediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than 90 in terms of the reduced plasma angle;

said second resonator circuit having a fundamental mode of resonance at a frequency higher than said center frequency; the number of said first and second resonator circuits being one each and the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with said input, said first, and said second resonator circuits being 50, and 35, respectively, in terms of the reduced plasma angle. 6. A velocity modulation tube operable in a predetermined operating passband of frequencies comprising at least one first resonator circuit and at least one second resonator circuit each located between an input and an output resonator circuit and a plurality of drift spaces for an electron beam passing throughall of said resonator circuits and said drift spaces, said second resonator circuit being placed downstream of said first resonator circuit with respect to said electron beam, each of said resonator circuits comprising interaction gap means operatively associated therewith and contributing to interaction between said electron beam and the electromagnetic field induced in the associated resonator circuit, selected ones of said drift spaces extending between the interaction gap means associated with two adjacent ones of said resonator circuits, said first resonator circuit having a fundamental mode of resonance at a frequency lower than the substantial center frequency of said passband, wherein the improvement is such that the normalized length of the drift space placed immediately downstream of the interaction gap means associated with said first resonator circuit is longer than the normalized lengths of the drift spaces places immediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than in terms of the reduced plasma angle;

said second resonator circuit having a fundamental mode of resonance at a frequency higher than said center frequency; the numbers of said first and second resonator circuits being one and two, respectively, and the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with said input, said first, one of said second, and the other of said second resonator'circuits being 50, 80, 45 and 330, respectively, in terms of the reduced plasma angle. 

1. A velocity modulation tube operable in a predetermined operating passband of frequencies comprising at least one first resonator circuit and at least one second resonator circuit each located between an input and an output resonator circuit and a plurality of drift spaces for an electron beam passing through all of said resonator circuits and said drift spaces, said second resonator circuit being placed downstream of said first resonator circuit with respect to said electron beam, each of said resonator circuits comprising interaction gap means operatively associated therewith and contributing to interaction between said electron beam and the electromagnetic field induced in the associated resonator circuit, selected ones of said drift spaces extending between the interaction gap means associated with two adjacent ones of said resonator circuits, said first resonator circuit having a fundamental mode of resonance at a frequency lower than the substantial center frequency of said passband, wherein the improvement is such that the normalized length of the drift space placed immediately downstream of the interaction gap means associated with said first resonator circuit is longer than the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than 90* in terms of the reduced plasma angle.
 2. A velocity modulation tube as claimed in claim 1, wherein said second resonator circuit has a fundamental mode of resonance at a frequency higher than said center frequency.
 3. A velocity modulation tube as claimed in claim 2, wherein said first resonator circuit has a fundamental mode of resonance at a frequency lower than the lower edge of said passband.
 4. A velocity modulation tube as claimed in claim 2, wherein said second resonator circuit has a fundamental mode of resonance at a frequency higher than the upper edge of said passband.
 5. A velocity modulation tube operable in a predetermined operating passband of frequencies comprising at least one first resonator circuit and at least one second resonator circuit each located between an input and an output resonator circuit and a plurality of drift spaces for an electron beam passing through all of said resonator circuits and said drift spaces, said second resonator circuit being placed downstream of said first resonator circuit with respect to said electron beam, each of said resonator circuits comprising interaction gap means operatively associated therewith and contributing to interaction between said electron beam and the electromagnetic field induced in the associated resonator circuit, selected ones of said drift spaces extending between the interaction gap means associated with two adjacent ones of said resonator circuits, said first resonator circuit having a fundamental mode of resonance at a frequency lower than the substantial center frequency of said passband, wherein the improvement is such that the normalized length of the drift space placed immediately downstream of the interaction gap means associated with said first resonator circuit is longer than the normalized lengths of the drift spaced placed immediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than 90* in terms of the reduced plasma angle; said second resonator circuit having a fundamental mode of resonance at a frequency higher than said center frequency; the number of said first and second resonator circuits being one each and the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with said input, said first, and said second resonator circuits being 50*, 80* and 35*, respectively, in terms of the reduced plasma angle.
 6. A velocity modulation tube operable in a predetermined operating passband of frequencies comprising at least one first resonator circuit and at least one second resonator circuit each located between an input and an output resonator circuit and a plurality of drift spaces for an electron beam passing through all of said resonator circuits and said drift spaces, said second resonator circuit being placed downstream of said first resonator circuit with respect to said electron beam, each of said resonator circuits comprising interaction gap means operatively associated therewith and contributing to interaction between said electron beam and the electromagnetic field induced in the associated resonator circuit, selected ones of said drift spaces extending between the interaction gap means associated with two adjacent ones of said resonator circuits, said first resonator circuit having a fundamental mode of resonance at a frequency lower than the substantial center frequency of said passband, wherein the improvement is such that the normalized length of the drift space placed immediately downstream of the interaction gap means associated with said first resonator circuit is longer than the normalized lengths of the drift spaces places immediately downstream of the interaction gap means associated with the other resonator circuits and is not longer than 90* in terms of the reduced plasma angle; said second resonator circuit having a fundamental mode of resonance at a frequency higher than said center frequency; the numbers of said first and second resonator circuits being one and two, respectively, and the normalized lengths of the drift spaces placed immediately downstream of the interaction gap means associated with said input, said first, one of said second, and the other of said second resonator circuits being 50*, 80*, 45* and 330*, respectively, in terms of the reduced plasma angle. 